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| United States Patent |
5,294,862
|
|
Banno
, et al.
|
*
March 15, 1994
|
Ladder-type piezo-electric filter
Abstract
In radio communication equipment, a ladder-type piezo-electric filter is
provided which operates in a high-frequency region of approximately 1.7
MHz-2 MHz. The filter is smaller than conventional ladder-type electric
filters, less costly, and more easily handled and produced. In accordance
with the present invention, rectangular piezo-resonators that
longitudinally generate harmonic oscillation are used in the series and
parallel branches of a ladder-type circuit. In each resonator,
piezo-electric regions are formed by dividing surface electrodes with
isolating slits. The slits are formed at the positions corresponding to
the node points of the harmonic oscillation waveforms. For the
piezo-resonators placed in the series branches of the ladder-type circuit,
the piezo-electric regions are polarized in the same transverse direction.
In each piezo-resonator used in a parallel branch, the piezo-electric
regions are polarized in alternately opposite transverse directions. In
addition, the divisional electrodes of the parallel branch
piezo-resonators are bridged over with a suitable conductor, such as
conductive paint, or the like.
| Inventors:
|
Banno; Hisao (Aichi, JP);
Nakagawa; Yasuhiko (Aichi, JP)
|
| Assignee:
|
NGK Spark Plug Co., Ltd. (Aichi, JP)
|
| [*] Notice: |
The portion of the term of this patent subsequent to December 1, 2009
has been disclaimed. |
| Appl. No.:
|
913702 |
| Filed:
|
July 16, 1992 |
Foreign Application Priority Data
| Current U.S. Class: |
310/366; 310/359; 333/189 |
| Intern'l Class: |
H03H 009/205; H01L 041/08 |
| Field of Search: |
310/359,366
29/25,35
333/187-192
|
References Cited
U.S. Patent Documents
| 3916490 | Nov., 1975 | Sheahan et al. | 20/25.
|
| 4287493 | Sep., 1981 | Masaie | 333/189.
|
| 4451753 | May., 1984 | Ogawa et al. | 310/315.
|
| 5077544 | Dec., 1991 | Ogawa et al. | 333/189.
|
| 5168253 | Dec., 1992 | Nakagawa et al. | 333/189.
|
| Foreign Patent Documents |
| 129840 | Feb., 1977 | DE | 333/189.
|
Primary Examiner: Voeltz; Emanuel T.
Assistant Examiner: LaBalle; C.
Attorney, Agent or Firm: Finnegan, Henderson, Farabow, Garrett & Dunner
Claims
What is claimed is:
1. A ladder-type piezo-electric filter circuit, comprising:
a first set of piezo-resonators connected to each other in series, each of
the first set of piezo-resonators including:
a rectangular piezo-electric element having an oscillation mode in a
longitudinal direction and having a first surface and an opposing second
surface and including at least two regions each polarized in the same
direction transverse to the first surface;
a first electrode on the first surface;
a second electrode on the second surface and divided into divisional
electrodes by at least one slit;
a second set of piezo-resonators connected to each other in parallel, each
of the second set of piezo-resonators including:
a rectangular piezo-electric element having an oscillation mode in a
longitudinal direction and having a first surface and an opposing second
surface and including at least two regions each polarized in alternately
opposite directions transverse to the first surface;
a first electrode on the first surface;
a second electrode on the second surface and divided into divisional
electrodes by at least one slit; and
conductors bridging the divisional electrodes of the second set of
piezo-resonators.
2. A ladder-type piezo-electric filter as recited in claim 1, wherein the
second electrode of each piezo-resonator in the first set and each
piezo-resonator in the second set is divided into two divisional
electrodes by one slit.
3. A ladder-type piezo-electric filter as recited in claim 2, wherein each
slit is formed at the position corresponding to the node point of a
waveform of two-times harmonic oscillation.
4. A ladder-type piezo-electric filter as recited in claim 1, wherein:
the first electrode and the second electrode of each piezo-resonator in the
first set is divided into two divisional electrodes by one slit; and
the second electrode of each piezo-resonator in the second set is divided
into three divisional electrodes by two slits.
5. A ladder-type piezo-electric filter as recited in claim 4, wherein the
slits are formed at the positions corresponding to the node points of a
waveform of three-times harmonic oscillation.
6. A ladder-type piezo-electric filter as recited in claim 1, wherein:
the first electrode of each piezo-resonator in the first set is divided
into two divisional electrodes by one slit;
the second electrode of each piezo-resonator in the first set is divided
into three divisional electrodes by two slits; and
the second electrode of each piezo-resonator in the second set is divided
into four divisional electrodes by three slits.
7. A ladder-type piezo-electric filter as recited in claim 6, wherein the
slits are formed at the positions corresponding to the node points of a
waveform of four-times harmonic oscillation.
8. A ladder-type piezo-electric filter as recited in claim 1, wherein:
the first electrode and the second electrode of each piezo-resonator in the
first set is divided into three divisional electrodes by two slits; and
the second electrode of each piezo-resonator in the second set is divided
into five divisional electrodes by four slits.
9. A ladder-type piezo-electric filter as recited in claim 8, wherein the
slits are formed at the positions corresponding to the node points of a
waveform of five-times harmonic oscillation.
10. A ladder-type piezo-electric filter as recited in claim 1, wherein the
width of each piezo-resonator in the first set differs from that of each
piezo-resonator in the second set.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a filter circuit for use in
radio communication equipment, such as portable mobile radio equipment, a
mobile telephone system, a cordless telephone, or the like, and
particularly relates to a ladder-type piezo-electric filter to be used in
a high-frequency region not lower than 1 MHz.
2. Discussion of the Related Art
Conventionally, ladder-type electric filters for radio communication
equipment have been designed to operate with a center frequency of 455
kHz. As shown in FIG. 12, such a filter typically has a configuration in
which piezo-resonators S, with resonance frequency at 455 kHz, and
piezo-resonators P, with antiresonance frequency at 455 kHz, are inserted
into series and parallel branches of a ladder-type circuit. Each
piezo-resonator has a pair of electrodes placed respectively on the front
and back surfaces of a square piezo-electric ceramic element. This filter,
however, has the disadvantage that its size is large, as each of the
piezo-resonators occupies an area of about 5 mm square.
To eliminate this disadvantage, a filter has been proposed which uses
piezo-resonators that generate longitudinal harmonic oscillation. Each
piezo-resonator has a pair of opposite electrodes placed respectively on
the front and back surfaces of a rectangular piezo-electric ceramic
element. Because its longitudinal or transverse size is at most 7/10 that
of the square piezo-resonator, this filter has the advantages of a smaller
size, a reduction in the amount of piezo-electric material used, and a
lower production cost.
For the radio communication equipment of mobile telephone systems, there is
a demand for a ladder-type piezo-electric filter of decreased size and
capability of operating in a high-frequency region of approximately 1.7
MHz-2 MHz. Such a filter requires resonators of small dimension in both
the longitudinal and transverse directions and, from the viewpoints of
production and handling, is difficult to realize.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above circumstances and
has as an object to provide a small-sized ladder-type piezo-electric
filter which is easily produced and which operates in a high-frequency
region.
A further object of the present invention is to provide a filter improved
in spurious characteristic.
Additional objects and advantages of the invention will be set forth in
part in the description which follows and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized and attained by means of
the instrumentalities and combinations particularly pointed out in the
appended claims.
To achieve the objects and in accordance with the purpose of the invention,
as embodied and broadly described herein, the ladder-type piezo-electric
filter, of this invention comprises rectangular piezo-resonators utilizing
longitudinal high harmonic oscillation inserted in the series and parallel
branches of the circuit. In each piezo-resonator, one of or both of the
opposite electrodes are divided into a certain number of divisional
opposite electrodes so as to generate predetermined high harmonic
oscillation. The resultant piezo-electric regions, which are sectioned off
by the divided opposite electrodes, are polarized. In the piezo-resonators
that are inserted in the series branches, the piezo-electric regions are
polarized in the same direction. For those inserted in the parallel
branches, the regions are alternately reversely polarized and the
divisional electrodes are bridged over with conductors.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute part of
this specification, illustrate embodiments of the invention and, together
with the description, serve to explain the objects, advantages and
principles of the invention. In the drawings,
FIG. 1 is a circuit diagram showing a ladder-type piezo-electric filter
according to an embodiment of the present invention;
FIGS. 2A and 2B are plan and cross-section views showing a rectangular
piezo-resonator which is to be inserted into a series branch of the
ladder-type circuit of FIG. 1 and which utilizes a two-times harmonic of
longitudinal oscillation;
FIGS. 3A and 3B are plan and cross-section views showing a rectangular
piezo-resonator which is to be inserted into a parallel branch of the
ladder-type circuit of FIG. 1 and which utilizes a two-times harmonic of
longitudinal oscillation;
FIG. 4 is a graph showing the impedance characteristic of each of the
series and parallel piezo-resonators;
FIG. 5 is a graph showing the filter characteristic of the ladder-type
circuit using series and parallel piezo-resonators according to the
present invention;
FIGS. 6A and 6B are plan and cross-section views showing a rectangular
piezo-resonator which is to be inserted into a series branch of the
ladder-type circuit of FIG. 1 and which utilizes a three-times harmonic of
longitudinal oscillation;
FIGS. 7A and 7B are plan and cross-section views showing a rectangular
piezo-resonator which is to be inserted into a parallel branch of the
ladder-type circuit of FIG. 1 and which utilizes a three-times harmonic of
longitudinal oscillation;
FIGS. 8A and 8B are plan and cross-section views showing a rectangular
piezo-resonator which is to be inserted into a series branch of the
ladder-type circuit of FIG. 1 and which utilizes a four-times harmonic of
longitudinal oscillation;
FIGS. 9A and 9B are plan and cross-section views showing a rectangular
piezo-resonator which is to be inserted into a parallel branch of the
ladder-type circuit of FIG. 1 and which utilizes a four-times harmonic of
longitudinal oscillation;
FIGS. 10A and 10B are plan and cross-section views showing a rectangular
piezo-resonator which is to be inserted into a series branch of the
ladder-type circuit of FIG. 1 and which utilizes a five-times harmonic of
longitudinal oscillation;
FIGS. 11A and 11B are plan and cross-section views showing a rectangular
piezo-resonator which is to be inserted into a parallel branch of the
ladder-type circuit of FIG. 1 and which utilizes a five-times harmonic of
longitudinal oscillation; and
FIG. 12 is a diagram showing the circuit of the conventional ladder-type
piezo-electric filter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, a piezo-electric filter according to the present invention is
shown in which rectangular piezo-resonators S.sub.1 and P.sub.1 are used.
Both resonators S.sub.1 and P.sub.1 generate longitudinal two-times
harmonic oscillation, and are inserted, respectively, into series and
parallel branches of a ladder-type circuit.
In accordance with the present invention, ladder-type piezo-electric
filters are provided with rectangular piezo-resonators. In each
piezo-resonator used in a series branch, for example, the resonators S in
FIG. 1, one of or both of the opposite electrodes are divided into a
certain number of divisional electrodes so as to generate a predetermined
high harmonic oscillation. A plurality of piezo-electric regions sectioned
by the divided opposite electrodes are polarized in the same direction.
In each piezo-resonator used in the parallel branches, for example, the
resonators P in FIG. 1, one of the opposite electrodes is divided into a
certain number of divisional electrodes so as to generate a predetermined
high harmonic oscillation. A plurality of piezo-electric regions sectioned
by the divisional opposite electrodes are alternately reversely polarized
and the divisional electrodes are bridged over with conductors.
A further feature of the ladder-type piezo-electric filter, according to
the present invention, is that the width dimension of the series-branch
piezo-resonators is different from that of the parallel-branch
piezo-resonators.
Each of the rectangular piezo-electric elements is characterized in that it
operates at a frequency not lower than 1 MHz. Further, when the with of
the series-branch piezo-resonator differs from that of the parallel-branch
piezo-resonator, spurious frequencies due to lateral resonance are shifted
from each other and thereby cancel each other. Thus, spurious frequencies
are reduced.
FIG. 2 illustrates the configuration of one rectangular piezo-resonator
S.sub.1 to be inserted into the series branch. In S.sub.1, opposite
electrodes 12 and 13 are formed by silver printing, or the like, on the
front and back parallel surfaces of a rectangular piezo-electric ceramic
element 11, the transverse size of which should not be larger than 7/10
times the longitudinal size. One of the opposite electrodes, electrode 12
on the front surface in this case, is halved into divisional electrodes
12a and 12b by an isolating slit 14. Slit 14 is formed at the position
corresponding to a node point n of the waveform (shown by a one-dot
chained line) of two-times harmonic oscillation. Two piezo-electric
regions 11a and 11b, one (11a) of which is between the divisional
electrode 12a and the back surface electrode 13, and the other (11b) of
which is between the divisional electrode 12b and the back surface
electrode 13, are subject to polarization treatment in the same direction
relative to thickness of the resonator as indicated by the arrows.
Further, the divisional electrodes 12a and 12b are connected to an
input-output line L.sub.1 of the ladder-type circuit.
FIG. 3 illustrates the configuration of one rectangular piezo-resonator
P.sub.1 to be inserted into the parallel branch. In P.sub.1, one of the
opposite parallel electrodes 22 and 23 of a rectangular piezo-electric
ceramic element 21, electrode 22 in this case, is halved by an isolating
slit 24 formed at the position corresponding to a node point n of
two-times harmonic oscillation. Further, the two piezo-electric regions
21a and 21b, one (21a) of which is between divisional electrode 22a and
the back surface electrode 23, and the other (21b) of which is between
divisional electrode 22b and the back surface electrode 23, are subject to
polarization treatment in opposite directions of thickness as indicated by
the arrows. The divisional electrodes 22a and 22b are bridged by a
suitable conductor 25, such as conductive paint or the like, and are
commonly connected to the input-output line L.sub.1 of the ladder-type
circuit. The back surface electrode 23 is connected to a ground line
L.sub.2.
The piezo-electric ceramic elements 11 and 21, that respectively constitute
resonators S.sub.1 and P.sub.1, have dimensions selected at 2.0 mm
(length).times.0.5 mm (width).times.0.25 mm (thickness) and are made, for
example, of lead zirconate titanate. The isolating slits 14 and 24, that
respectively halve front surface electrodes 12 and 22, are of 0.3 mm
width. The piezo-electric regions 11a, 11b, 21a and 21b were polarized in
the predetermined directions with a DC voltage of 3 kV/mm. FIG. 4
demonstrates the results of measurements of the impedance characteristic
of the series and parallel resonators S.sub.1 and P.sub.1. The resonance
f.sub.l of large resonance impedance was caused at the frequency 1.7 MHz
of an aimed two-times harmonic.
Further, four series resonators S.sub.1, in which the resonance frequency
was set at 1.7 MHz and the electrostatic capacity was set at 25 pF, and
four parallel resonators P.sub.1, in which the antiresonance frequency was
set at 1.65 MHz and the electrostatic capacity was set at 90 pF, were
wired so as to constitute the ladder-type circuit of FIG. 1. The
input-output resistors R.sub.1 and R.sub.2 were connected so that R.sub.1
=R.sub.2 =2k.OMEGA.. FIG. 5 illustrates the measured filter characteristic
of the resultant circuit.
Although the above embodiments have been described using the series and
parallel resonators S.sub.1 and P.sub.1 which generate two-times harmonic
oscillation, rectangular resonators which generate three-times or more
harmonic may be used for the same purpose. FIGS. 6-10 exhibit the
necessary modifications.
FIGS. 6 and 7 respectively illustrate series and parallel resonators
S.sub.2 and P.sub.2 which utilize three-times harmonic oscillation. FIGS.
8 and 9 similarly illustrate series and parallel resonators S.sub.3 and
P.sub.3 which utilize four-times harmonic oscillation. Lastly, FIGS. 10
and 11 respectively represent series and parallel resonators S.sub.4 and
P.sub.4 which utilize five-times harmonic oscillation.
In the series resonator S.sub.2, as shown in FIG. 6, isolating slits 14
define the positions of node points n corresponding to the higher harmonic
oscillation. The slits are formed on the front and back surfaces of the
electrodes. The respective piezo-electric regions are connected in series
to an input-output line L.sub.1.
In the series resonators S.sub.2, S.sub.3, and S.sub.4, illustrated
respectively in FIGS. 6, 8, and 10, the isolating slits 14 divide the
respective front surface electrodes 12 into divisional electrodes 12c-12j.
The back surface electrodes 13 are divided by isolating slits 14 into
divisional electrodes 13a-13g. The result is the formation of
piezo-electric regions 11c-11n, whose directions of polarization are shown
by the arrows.
In the parallel resonators P.sub.2, P.sub.3, and P.sub.4, illustrated
respectively in FIGS. 7, 9, and 11, the front surface electrodes 22 are
divided by isolating slits 24 into divisional electrodes 22c-22n. The
resultant piezo-electric regions 21c-21n are polarized in the directions
indicated by the arrows.
All other reference numerals used in FIGS. 6-11 designate the same portions
as those in FIGS. 2 and 3.
FIG. 4, which exhibits the impedance characteristic of the rectangular
resonator, further illustrates the lateral resonance frequency, or
spurious frequency, f.sub.2 that is generated in addition to the
longitudinal main resonance frequency f.sub.1. Because the lateral
resonance frequencies of the respective resonators are substantially equal
to each other, the problem is presented that when using a plurality of
resonators having the same width, resonance is generated producing an
intense spurious wave. However, when the width of the respective
rectangular resonators is different from each other, the lateral resonance
frequencies are shifted, causing them to interfere or cancel each other
and, thereby, suppress the spurious frequencies.
As described above, a ladder-type piezo-electric filter can be obtained
that operates at a predetermined frequency not lower than 1 MHz and
utilizes rectangular piezo-resonators with a high harmonic of longitudinal
oscillation. The division of the opposite electrodes of the
piezo-resonators into a certain number of divisional electrodes and the
polarization of the resultant piezo-electric regions generate a high
harmonic of longitudinal oscillation. Further, when the width of the
rectangular piezo-resonators is made different from each other, it is
possible to provide a filter improved against spurious frequencies.
Because the rectangular resonators are smaller than square resonators and
utilize less material, it is possible to obtain a small-size, inexpensive
and high frequency ladder-type piezo-electric filter capable of more
simplified handling, production, and assembly.
The foregoing description of preferred embodiments of the invention has
been presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise form
disclosed, and modifications and variations are possible in light of the
above teachings or may be acquired from practice of the invention. The
embodiments were chosen and described in order to explain the principles
of the invention and its practical application to enable one skilled in
the art to utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It is
intended that the scope of the invention be defined by the claims appended
hereto, and their equivalents.
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